The present invention relates to a micromirror device configured to tilt a mirror in minute quantity by causing electrostatic attraction between adjacent electrodes.
Recently, various types of micro devices are in practical use with development of MEMS (Micro Electro Mechanical Systems) technology. Among such micro devices is a micromirror winch can be used, for example, as a scanner adapted for a barcode reader, a laser printer, and etc.
U.S. Pat. No. 5,959,760 discloses some types of micromirrors, The micromirror disclosed in this publication is formed to be an electrostatic driving type configured to tilt a mirror in minute amount by electrostatic attraction acting between electrodes. A first example of a micromirror disclosed in U.S. Pat. No. 5,959,760 is configured to locate electrodes (movable electrodes) at both sides of a mirror and to locate fixed electrodes adjacently to the both sides of the mirror.
In the first example, the two fixed electrodes are located at positions slightly upper than a position of the mirror (i.e. positions of the movable electrodes). In order to tilt the mirror, first, only one movable electrode at one side of the mirror is supplied with a voltage. At this time, electric charges of opposite polarities are respectively accumulated on a surface of the movable electrode being supplied with the voltage and a surface of a neighboring fixed electrode. Thus, electrostatic attraction acts between these surfaces of the movable electrode and the fixed electrode. Then, the movable electrode is drawn toward the fixed electrode and thereby the mirror is tilted. If a voltage is applied to the other movable electrode, the mirror tilts in a direction opposed to the above mentioned tilting direction, By alternately applying a voltage to one of the movable electrodes, the mirror swings.
A fifth example of a micromirror disclosed in U.S. Pat. No. 5,959,760 is configured such that one of fixed electrodes is located at a position slightly upper than a position of a mirror and the other fixed electrode is located at a position slightly lower than the position of the mirror. By simultaneously applying a voltage to both the fixed electrodes located adjacently to the sides of the mirror, electrostatic attraction is caused at both of an interval between one of the fixed electrodes and one of the movable electrodes and an interval between the other fixed electrode and the other movable electrode so that the mirror rotates in the same direction. According to the fifth example, a driving force corresponding to the double of the electrostatic attraction caused in the first example can be obtained.
A sixth example of a micromirror disclosed in U.S. Pat. No. 5,959,760 is configured such that, at each side of a mirror, a pair of fixed electrodes is provided so that each side of the mirror is situated in a gap between the pair of fixed electrodes. By applying a voltage to the fixed electrodes as in the case of the fifth example, it is possible to drive the mirror by electrostatic attraction corresponding to the double of the electrostatic attraction generated in the micromirror of the first example. It is also possible to generate electrostatic attraction to rotate the mirror in one of normal rotation and inverse rotation. Furthermore, according to the sixth example, it is possible to tilt the mirror by an angle larger than that achieved in the other examples.
With regard to a micromirror, there is a demand for securing a sufficient tilting angle and a sufficiently strong driving force. As described above, the micromirror in the first example of U.S. Pat. No. 5,959,760 is able to generate the electrostatic attraction enabling the mirror to tilt in both of the directions of normal rotation and inverse rotation. Therefore, according to the first example, a relatively large tilting angle of a mirror can be secured. However, the micromirror of the first example has a drawback that a relatively strong driving force can not be secured because the micromirror generates the electrostatic attraction only at one side of the mirror.
Furthermore, because the electrostatic attraction is generated at one side of the mirror, an unbalanced load may be put on a structural component (e.g., a torsion bar) in the micromirror. Such an unbalanced load may cause the structural component to deform in an unexpected direction, thereby decreasing the durability of the structural component. Since the unbalanced load may also cause loss of energy, the efficiency of conversion from the electrostatic attraction to the rotation of the mirror may be decreased.
As described above, the micromirror in the fifth example of U.S. Pat. No. 5,959,760 is configured to generate the electrostatic attraction at both sides of the mirror to rotate the mirror in the same direction. Therefore, according to the fifth example, a relatively strong driving force can be secured. Since the electrostatic attraction acts symmetrically with respect to a torsion bar in the micromirror, an unbalanced load is not put on the structural component (e.g., the torsion bar) of the micromirror (or an unbalanced load can be decreased).
However, the micromirror of the fifth example has a drawback that it is able to rotate the mirror only in one direction, and therefore a relatively large tilting angle of the mirror can not be secured.
On the other hand, the micromirror in the sixth example of U.S. Pat. No. 5,959,760 is able to secure a relatively large tilting angle of the mirror and a relatively strong driving force. However, since the fixed electrodes need to be positioned on the upper side and the lower side of the movable electrode, the structure of the micromirror becomes considerably complicated, which may reduce production efficiency of micromirrors (i.e., reduce a yield of micromirrors or increase a lead time of production of micromirrors).